BS (Broadcasting Satellite) digital broadcasting provides services such as audio, video, and data broadcasting in a plurality of levels such as a higher-level layer service and a lower-level layer service. For example, the higher-level layer service uses 8PSK (8 Phase Shift Keying) as its modulation method, and the lower-level layer service uses QPSK (Quadrature-PSK) or BPSK (Binary-PSK) as its modulation method.
A digital broadcast receiver receives a BS digital broadcasting service, and if it becomes unable to sufficiently receive a higher-level service because a reception CNR (Carrier-to-Noise Ratio) is degraded by rain or the like, it switches to a lower-level layer service to prevent the service from being intercepted.
For example, if the higher-level layer service is for HD (High Definition) video signals and the lower-level layer service is for audio signals and the reception CNR degrades to such an extent that the higher-level layer service cannot be received sufficiently, a conventional digital broadcast receiver freezes or shuts down the HD video signals of the higher-level layer service. After the HD video signals are frozen or shutdown, the conventional broadcast receiver executes a pre-stored application and displays, for example, a message to the effect that the higher-level layer service cannot be received, on a television receiver or the like.
In this case, the conventional digital broadcast receiver changes the received transport stream packet (hereinafter called a TS packet) for the higher-level layer service to a null packet. In this manner, the performance of receiving the lower-level layer service can be prevented from being degraded.
Namely, the conventional digital broadcast receiver detects a PID (packet ID) allocated to a received TS packet and corresponding to a desired program number and separates the TS packet into an audio PES (Packetized Elementary Stream) and a video PES. When the lower-level layer service is to be received, the conventional digital broadcast receiver changes the TS packet of the higher-level layer service to a null packet in accordance with transport and multiplexing configuration control (TMCC) information read from a received signal, and thereafter separates the TS packet into the audio PES and video PES. Even if the received data representative of PID has errors unable to be corrected, it is therefore possible to prevent the TS packet of the higher-level layer service from being extracted as the TS packet of the lower-level layer service.
However, the conventional digital broadcast receiver cannot receive the higher-level layer service while it receives the lower-level layer service because of rain or the like. It is therefore impossible to satisfy the user requirement of receiving the higher-level layer service even if the reception CNR is degraded.
If the audio PES and video PES are separated without changing the TS packet of the higher-level layer service to the null packet while the lower-level layer service is received, the lower-level layer service is adversely affected and cannot be received reliably although it can be received otherwise normally.
The present invention has been made under such circumstances and aims to provide a digital broadcast receiver capable of adequately receiving the higher-level layer service along with the lower-level layer service even if a reception CNR is degraded.